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Full text of "Handbook Of Chemical Engineering - I"

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CRYSTALLIZATION                                      401
developed, this process is today carried out in many parts of the world in exactly . the same manner as it has been for thousands of years. Shallow tanks or reservoirs, often of enormous size, are pumped full of sea water, which is allowed to concentrate naturally until the crystals of salt begin to form and collect in considerable quantities, when they are skimmed out and a new crop allowed to form, to be removed in turn, the process being allowed to continue until some of the other salts begin to separate, when the mother liquor is run off and the tank refilled.
Since the evaporation is mainly due to the heat of the sun the process is largely confined to the warm countries where the percentage of bright days is high. This is practically the only process of crystallization which is carried out without artificial assistance and is of special interest not only for that reason but also from the fact that it is a most beautiful example of purification by crystallization, there being a large number of salts other than sodium chloride in sea water. In this process of natural crystallization the salt crystals of high purity are produced due to the fact that the combination of other salts is somewhat more soluble than the sodium chloride, and that there is a greater amount of it, so that it accordingly crystallizes out first as the concentration proceeds.
The production of sodium chloride by the evaporation of natural brines is of great antiquity in China. These brines contain very small amounts of impurities as compared to sea water and in Chinese practice are generally evaporated to solid cakes in iron pans. In most other countries, however, the demand is for a salt of high purity. The brines are therefore concentrated in some form of evaporator and the crystallization allowed to take place progressively, the crystals being worked in one direction and mother liquors in the other. Several grades of salt are made for various uses according to the degree of purity desired. The purest crystals go for table and dairy use, others for cattle and other stock, for manufacturing, etc., down to the most impure grades which are made from the final mother liquors largely for use in freezing mixtures for ice cream, etc.
In this country within the last few years an important improvement in process increases the yield of high purity crystals at the expense of those of low purity. Many salt wells contain measurable quantities of barium salts,;which are decidedly poisonous and have caused much loss through the use for stock of low purity salt containing salts of barium. The improvement in process is purification by .precipitation of impurities. The brine as it comes from the wells receives a calculated quantity of salt cake (acid sodium sulphate), which, being a byproduct of nitric acid manufacture, is very cheap. The sulphuric-acid radical combines with the barium salts to form the insoluble barium sulphate, which is removed by a settling process and the practically barium-free brine concentrated as usual. Crystallization by concentration has been treated thus fully because it is a somewhat rare case.
Sodium chloride is peculiar in that it is little more soluble in hot water than in cold. Salts in general show greater solubility as the temperature of the solution rises though there are cases, notably that of some calcium salts, in which the solubility decreases as the temperature rises. The first case is so generally true that crystallization by concentration and by cooling may be regarded as steps of the same process. However, crystallization by cooling alone has its own distinct place as will be described later.
Crystallization by Concentration and Cooling.—There are very many cases in chemical manufacture where the desired product is produced as a practically pure water solution of the substance. This is especially true of metallic salts such as ferrous sulphate, copper sulphate, zinc sulphate, lead acetate, etc. The solutions of these salts are generally concentrated in open evaporators, usually